1. Field of the Invention
This invention relates to a means for reducing the sound energy which is reflected from or radiated by an underwater structure. More specifically, it relates to a means for absorbing sound energy in an elastomeric coating, which in addition to dissipating energy through hysteric mechanisms, makes use of the piezoelectric or magnetostrictive mechanism to convert sound energy to electric or magnetic energy which is then converted to heat by resistive elements, which may be either internal or external to the absorptive layer. Further, is relates to providing desired acoustic performance levels in a one or two decade frequency band with coatings which are one-half to one-third as thick as prior designs. It also provides acoustic performance insensitive to hydrostatic pressure variations of one decade or more.
2. Description of the Prior Art
The primary means currently for preventing the reflection and radiation of sound from underwater structures is through use of anechoic or antiradiative coatings constructed of viscoelastic rubbers which dissipate energy through hysteric losses. The coating is installed on the exterior of the structure thereby forming a layer between the surrounding water and the structure's outer surface. The coating design allows substantially matching the impedance of the structure presented to the waterborne sound wave to the characteristic impedance of water, p.sub.o C.sub.o where p.sub.o is the density and c.sub.o is the sound speed of water, thus allowing the sound to pass directly into the coating without significant amounts of the energy being reflected. The longitudinal sound waves, after passing into the coating, are converted into shear deformations by air voids purposely formed in the rubber layer during its manufacture. The energy within the shear deformations is dissipated by the hysteric losses present in the properly formulated viscoelastic polymer. Other means for absorbing underwater sound energy is through use of piezoelectric or ferroelectric effect in single layer coatings as illustrated in U.S. Pat. Nos. 3,515,910 and 3,614,992.